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The Cambrian Period


Abstract

This page describes the Cambrian Period, including stratigraphy, the fossil record, and the phenomenon known as the “Cambrian explosion.” Three famous lagerstätten – Chengjiang, Sirius Passet and the Burgess Shale – are briefly discussed, followed by a sketched outline of some of the major fossil groups.

Keywords: Cambrian, Cambrian biota, fossil record, evolution, Metazoan radiation

Introduction

In the sediments of Cambrian age, fossils ‘suddenly’ become common for the first time. This effect, which has come to be known as the “Cambrian Explosion,” has guaranteed the Cambrian Period a special place in the hearts and minds of generations of paleontologists.

Understanding of Cambrian fossils, and their evolutionary significance, has been hugely enhanced by the study of fossils from three famous lagerstätten. Although the Middle Cambrian Burgess Shale had been known since the early 1900s, very detailed study would not be accorded these fossils until the mid 1970s. The definitive popular account of this research, together with a contentious interpretation, is to be found in Stephen J. Gould’s book, Wonderful Life. More recently, in the 1980s and ’90s, significantly older Early Cambrian lagerstätten have been located and studied at the Chengjiang and Sirius Passet localities in China and Greenland, respectively.

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Further Reading

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Stratigraphy

Type Section/Sections

“Adam Sedgwick named the Cambrian in 1835. He derived the name from ‘Cambria,’ the Roman name for Wales, site of the type area, in which shales and sandstones make up a section about two miles [~3 km] thick. Though these rocks are strongly folded and faulted, some are fossiliferous” (Stirton 1967, p. 68).

In 1831 Sedgwick and a collaborator, Sir Roderick Impey Murchison, commenced work on the stratigraphy of northern Wales. Sedgwick began at the bottom of the section and Murchison at the top. Sedgwick named his sequence of rocks the Cambrian and Murchison applied the name Silurian to the generally more fossiliferous upper formations. Eventually their sections overlapped, each claiming some of the same rocks for their systems, in what became a widespread and lengthy controversy until, forty four years later, Charles Lapworth introduced the name Ordovician for the disputed sequence.

The type locality is in western Wales and is defined mainly by lithology.

The Lower (Vendian–Cambrian) Boundary

Since 1947, when H.E. Wheeler initiated debate with the suggestion that the Precambrian-Cambrian boundary should be based upon the first appearance of trilobites, much has ensued. Progress has largely been facilitated by the International Geological Congress (IGC) and the establishment in 1960 of a Subcommission on Cambrian Stratigraphy. The classical idea of placing the boundary at an unconformity was displaced by the search for monofacial, continuous deposition sequences across the boundary, with the view to selecting a stratotype. This search produced a wealth of data from around the world – including the Palaeotethyan Belt, Siberian Platform, and England – eventually focusing upon south-east Newfoundland where the stratotype is now located.

Prior to 1990, the boundary was generally placed at the base of the Tommotian. However, in 1991 the International Subcommission on Cambrian Stratigraphy (through its Working Group on the Precambrian-Cambrian Boundary) made the official decision to draw the base on the Cambrian at the first appearance datum (FAD) of the distinctive horizontal burrow ichnofossil, Trichophycus (formerly Phycodes) pedum (Seilacher 1955, fig. 1), in the reference section at Fortune Head. This horizon correlates with the base of the Siberian Nemakit-Daldynian Stage, some 13 Ma earlier than the Tommotian, included within the Vendian in some older literature.

Trichophycus pedum (28472 bytes)

Fig. 1: The horizontal burrow trace fossil, Trichophycus (formerly Phycodes) pedum defines the lower boundary of the Cambrian in the reference section at Fortune Head, southeastern Newfoundland. [Image courtesy of Dr. Gerd Geyer, Institut für Paläontologie, Bayerische Julius-Maximilians-Universität, Würzburg, Germany.]

The Upper (Cambrian–Ordovician) Boundary

The Cambrian was brought to a close by a ‘mass extinction’ event which notably decimated the trilobites. (Read more.)

Here, too, steadily accumulating data has led to similarly changing opinions as to where the boundary should be drawn. “Traditional concepts of the Cambrian-Ordovician boundary [were] based on the occurrence of the graptolite Dictyonema (formerly Rhabdinopora) flabelliforme (Eichwald 1840), which has a limited regional distribution. Strata with Dictyonema flabelliforme are often difficult to correlate precisely so that different species (and subspecies) of conodonts were favoured for definition of the Cambrian-Ordovician boundary. The Cambrian-Ordovician Boundary Working Group finally decided in 1998 by majority that the base of the Ordovician should be placed at the base of the zone with Iapetognathus fluctivagus, which approximates the Cordylodus lindstromi Zone, the Dictyonema flabelliforme Zone and the FAD of the trilobite Jujuyaspis. The GSSP for this boundary was chosen at the Green Point section, Newfoundland” (International Subcommission on Cambrian Stratigraphy web page).

The age is established at 490 Ma.

Current Chronology of the Cambrian

Until 1991, when the base of the Cambrian was finally formally defined, the base of the Period was most often taken as the base of the Tommotian Epoch (and one still finds that, commonly). The new definition added the Nemakit-Daldynian (or Manykaian) below the Tommotian, enlarging the Period by approximately a half.

Nevertheless, recent geochronological studies have provided absolute age estimates which actually reduced the understood duration of the Cambrian. Thus, these studies have reinforced the impression that the events of the Cambrian occurred very quickly – of a “big bang” of animal evolution – by narrowing the available time, during which the apparent divergences appeared in the fossil record, to just a few million years.

“Fossils are found in sedimentary rocks; these rarely contain minerals that both formed when the sediment was deposited and also have the right kinds of isotopes used for age determination. It has therefore been necessary to use isotopic dating techniques that are less certain than those applied to igneous and metamorphic rocks. As a result, the assumed age of the ... Precambrian-Cambrian boundary has fluctuated with the method of analysis. The latest approach – using an ion probe to measure uranium and lead (206Pb/238U and 207Pb/206Pb) in single crystals of zircon from volcanic ash beds – yields ages that are significantly younger than those obtained by other methods.... Thus the ‘textbook’ age of the Precambrian-Cambrian boundary has recently been reduced from 570-590 Ma to 530-550 Ma [the International Subcommission on Cambrian Stratigraphy has, for the present, settled on an age of 543 Ma for the Precambrian-Cambrian boundary]. This revision towards a younger age, if correct, is of some importance because the end of the Cambrian appears to be no younger than about 500-510 Ma [now settled at approximately 490 Ma]. Consequently, the Cambrian radiation of the Metazoa may have been compressed into a much shorter interval of time than previously considered possible – perhaps as little as ten million years” (Runnegar 1992, p. 70).

CamChron.gif (4321 bytes)

Fig. 2: Our view of the chronology of the Cambrian has changed over time, with the overall trend being a reduction in the supposed length of the Cambrian from almost 100 million years to just 53 Ma.

The International Subcommission on Cambrian Stratigraphy has settled on an age of 543 Ma for the Precambrian-Cambrian boundary, and approximately 490 Ma for the Cambrian-Ordovician boundary. These are the values adopted throughout this site. (Read more.)

Paleogeography

Major Tectonic Events

The Precambrian supercontinent variously known as Proto-Pangea, Ur-Pangea and Rodinia formed ~1,000 Ma from the amalgamation of three or four pre-existing continents, in an event known as the Grenville Orogeny. Perhaps beginning ~700 Ma, but protracted over many millions of years, Rodinia began breaking up into three major blocks: West Gondwana, East Gondwana, and Laurasia. Subsequently - perhaps ~540 Ma, near the beginning of the Cambrian - West and East Gondwana merged in the mountain-building event known as the Pan-African Orogeny. (After Rogers 1996.)

Land and Sea

Gondwana, comprising much of what is now India, Australia, New Zealand, Antarctica, Africa and South America, was a super-continent stretching from ~35ºN almost to the South Pole.

North America and parts of Scotland were united in the subequatorial continent of Laurentia, surrounded by calm seas allowing the accumulation of extensive limestone deposits in marginal basins. Approximately 505 Ma ago, a mudslide on the western continental margin of Laurentia, in what is now eastern British Columbia, buried the famous Burgess Shale fauna.

Other land masses included Baltica, comprising much of modern western Europe, also near the South Pole, and Siberia, which occupied equatorial latitudes.

The Iapetus Ocean stretched between Laurentia and Baltica; present day Greenland and Scandinavia occupying the continental margins of Laurentia and Baltica, respectively. The north and south of present day China were separated, lying beneath shallow seas in northern tropical latitudes.

The evidence concerning north Africa in Early Cambrian times is equivocal: paleomagnetic studies indicate a position near the South Pole, adjacent to the south of Gondwana, yet the faunas, which include coral reefs, are tropical.

Avalonia, comprising Wales and parts of England, New Foundland, New England and Nova Scotia, was also submarine at this time, and near the South Pole.

(RCB paleogeographic reconstruction)

Climate

Although ice ages both preceded (Varanginian) and followed the Cambrian, during Cambrian times there was no significant ice formation. Global climate was probably warmer and more uniform than it is today.

Paleontology

General Characteristics

Stromatolites are widely distributed in Cambrian carbonates, as are the calcified remains of photosynthetic organisms. Siliciclastic rocks yield abundant organic-walled microfossils, and many lithologies preserve a biogeochemical record of microbial diversity. Radiolaria, udoteacean green algae, and calcareous red algae may have evolved during the Cambrian, but they radiated during the Ordovician. (After Knoll 1996.)

Earliest Cambrian assemblages contain few metazoan taxa, though, and the diversity of trace and body fossils grew only over a protracted interval.

However, the basal Cambrian may play host to more animal diversity than is currently recognised: Although reports of fossilised eggs and other early developmental stages of marine invertebrates are rare, probably due to the difficulties of recognizing them, there is an abundance of small globular structures in the fossil record, leading up to and including the earliest Cambrian. In 1994, Zhang and Pratt reported Middle Cambrian spherical fossils, 0.3 mm in diameter, that under a smooth membrane preserved a polygonal pattern which they interpreted as remains of blastomeres belonging to 64- and 128-cell stages of arthropod embryos. Shortly after, in 1997, Bengtson & Zhao reported similar occurrences of globular fossils from basal Cambrian rocks. Moreover, their finds included eggs containing identifiable embryos of metazoans (fig. 3) including a probable arthropod (also see below).

“[S]tudies of Cambrian fossiliferous strata in Siberia, China, Europe, and Australia have yielded an unexpected range of well-preserved phosphatic microfossils. Most are either tiny shells or the disarticulated components (sclerites) of a protective armour comprised of many parts (Bengtson et al. 1990). These microfossils have become familiar as the ‘small shelly fossils.’ Most are original phosphatic skeletons or phosphatic copies of other kinds of hard parts ...” (Runnegar 1992 p. 66).

“Hyoliths and halkierids (extinct forms thought to be related to mollusks), true conchiferan mollusks and, perhaps, chaetognaths enter the record during the first 10 to 12 million years of the Cambrian, but crown-group fossils of most other bilaterian phyla appear later: the earliest body fossils of brachiopods, arthropods, chordates, and echinoderms all post-date the beginning of the period by 10 to 25 million years” (Knoll & Carroll 1999).

“[A]lmost all major bilaterian clades already are represented in the Lower Cambrian, in remarkably preserved deposits such as the Chengjiang fossil lagerstätte. Fossil remains of diverse bilaterian forms from the Lower Cambrian have been obtained from many other regions of the globe as well. The latest Precambrian also has yielded trace fossils of unmistakable bilaterian origin. These remains indicate that major evolutionary diversification of animals had occurred by the onset of the Cambrian, and, therefore, more remote ancestral forms must have been alive earlier” (Chen et al. 2000, p. 4457).

(A) Indet. embryo (14261 bytes)     (B) Olivooides multisulcatus (20303 bytes)

Fig. 3: (A) Reproduction of fig. 1A from Bengtson & Zhao 1997, a SEM image depicting a suggested metazoan embryo – possibly Olivooides multisulcatus – at approximately the 256-cell stage. Sample is NGMC (National Geological Museum of China) 9351 from the basal Cambrian Dengying Formation exposed in the Shizhonggou section, Shaanxi Province, China; scale is 500µm.

(B) Reproduction of fig. 1K from Bengtson & Zhao 1997, a SEM image of the cnidarian Olivooides multisulcatus. Sample is CAGS (Chinese Academy of Geological Sciences) 32372, also from the Dengying Formation exposed in the Shizhonggou section; scale is the same as A.

As a whole, the Cambrian metazoan fossil record is dominated numerically by arthropods, chiefly trilobites, of which there are over 100 Cambrian families. Next, but much lower in abundance, are Brachiopoda, represented chiefly by a number of inarticulate orders that may prove to merit class status. Hyoliths and certain members of the Mollusca, represented chiefly by monoplacophorans, form important though numerically minor elements.

The Nemakit-Daldynian (543 to 530 Ma)

In many ways this is potentially the most fascinating interval of all: the Ediacarans have essentially gone, yet the diverse range of Tommotian SSFs have yet to appear. Melodramatic as it may sound, I cannot resist the phrase "calm before the storm." Just what was happening around this time?

The type section lies within the Nemakit-Daldyn Formation (which ranges up into the Tommotian) from the west Anabar region on the northern Siberian Platform, northwest of Olenek in the Russian Federation. Biostratigraphically, it is the equivalent of the Anabarites trisulcatus Zone. (For further reading, see Brasier et al. 1997, p. 305.)

"Although most Ediacaran fossils have no post-Proterozoic record, they were not immediately succeeded in lowermost Cambrian rocks by diverse crown group bilaterians. Earliest Cambrian assemblages contain few taxa, and the diversity of trace and body fossils grew only over a protracted interval. Hyoliths and halkierids (extinct forms thought to be related to mollusks), true conchiferan mollusks and, perhaps, chaetognaths [possibly represented by protoconodonts] enter the record during the first 10 to 12 million years of the Cambrian, but crown-group fossils of most other bilaterian phyla appear later: the earliest body fossils of brachiopods, arthropods, chordates, and echinoderms all post-date the beginning of the period by 10 to 25 million years. Trace fossils suggest earlier appearances for some groups, notably arthropods, but the observation remains that the Early Cambrian contains considerable time for the assembly and diversification of crown group morphologies" (Knoll & Carroll 1999, p. 2131).

Lagerstätten

Lagerstätten (sing. lagerstätte) are fossil localities which are highly remarkable for for either their diversity or quality of preservation; sometimes both. There are three famous Cambrian lagerstätten – Sirius Passet, Chengjiang and the Burgess Shale – and a handful of other localities of special merit.

Sirius Passet: Located on the northern coast of Greenland, this lower Cambrian site has yielded a diverse though not spectacularly preserved fauna, including a Redlichiid trilobite, Buenellus higginsi, the halkieriid Halkieria evangelista, and the enigmatic lobopod Kerygmachela kierkegaardi. The age is around 515 to 520 Ma.

Chengjiang: Fossils from Chengjiang, near the city of Kunming in Yunnan Province, China, preserve a diverse biota of rather similar age to the Sirius Passet fauna; the differences between their fossil assemblages are attributed to environment and geography. The soft-bodied fossils include algae, medusiform metazoans, chondrophorines, sponges, chancelloriids, sea anemones, priapulid worms, hyoliths, possible ectoprocts, inarticulate brachiopods, annelid-like animals, lobopodians, trilobites and other arthropods, hemichordates, chordates as well as taxa that cannot definitely be assigned to any well established groups.

Emu Bay Shale (Kangaroo Island): The Lower Cambrian Emu Bay Shale of Kangaroo Island, South Australia, has long been famous as a source of magnificent specimens of the trilobites Redlichia takooensis and Hsuaspis bilobata. It is additionally important as the only site in Australia so far to yield a Burgess Shale-type biota. As such it represents an important basis for comparison with other Burgess Shale-type assemblages, particularly Chengjiang, which is the closest paleogeographically, although somewhat older.

Burgess Shale: The Burgess Shale is treated exhaustively almost everywhere and need not be revisited here. Some links are provided on the following page:

(More Cambrian Lagerstätten.)

Major Evolutionary Events

Life on earth has a long and rich history – predating the beginning of the Cambrian by a factor of six or seven. However, much of this early life was microscopic and, microscopic or not, virtually unknown until the 1960s. Prior to then, as far as was widely known even among the scientific community, the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and marvellous diversity: the so-called “Cambrian Explosion.”

Charles Darwin (Origin of Species – 2nd ed. Chapter IX) recognized that the sudden appearance of animal fossils in the Cambrian posed a problem for his theory of natural selection, “and may be truly urged as a valid argument against the views here entertained.” He suggested that “before the lowest [Cambrian] stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the [Cambrian] age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.”
Today, although the fossil record now extends back 3,465 Ma (± 5 Ma, to the Apex Chert, see Schopf 1999, p. 100) and diverse precursors to the Cambrian biotas are gradually becoming understood, it still appears as if a genuinely rapid diversification of form, particularly among the Metazoa (“animals”), did occur in the Cambrian. During this time, most extant body plans are suddenly found in the fossil record. “Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla (Valentine et al. 1991)” (Wray et al. 1996). By way of contrast, “it appears that no [new] phylum-level body plans have arisen in the animal kingdom in the last 500 million years” (Arthur 1997, p. 7).
When this radiation began, and how rapidly it unfolded, is still the subject of on-going research, yet much has become clear since Darwin’s day. (Read more.)

Major Taxa

The most widely known of the ‘cast of characters’ from the Cambrian are probably the arthropods. Gould 1989 is largely responsible for the popularisation of the Cambrian Arthropoda, especially the problematic forms, since reinforced by the likes of Briggs et al. 1994 and Conway Morris 1998, although anyone with the most casual interest in paleontology would have been aware of at least the Trilobita before the publication of any of those books. The Cambrian wasn’t all exoskeletons, however. In fact, nearly all modern phyla are known as fossils from the Cambrian. Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla.

Sponges are generally accounted the most primitive of Metazoans, although probable cnidarians pre-date them in the fossil record. Both were well-established long before the Cambrian and are known throughout the period. Sponges are a major component in Lower Cambrian Chengjiang fauna, where they are second only to arthropods in both generic and specific diversity. About 1000 sponge body fossils, distributed among 15 genera and 30 species, have been collected there.
“Archaeocyaths are an extinct group of sponges that had a very brief (geologically speaking) and spectacular history. The first archaeocyaths appear roughly 530 million years ago, during the Lower Cambrian. Archaeocyath species were very important members of Lower Cambrian communities. They diversified into hundreds of species during this time period and some of these species contributed greatly to the creation of the first reefs. Reef ecosystems tend to support a wide variety of organisms both in the present and in the past. Despite their great success in terms of numbers, the archaeocyaths were a short-lived group. They were almost completely non-existent by the middle Cambrian, some 10 to 15 million years after their first appearance” (Berkeley web site - read more).
Probable cnidarians are known from Precambrian sediments of the Twitya Formation in the Mackenzie Mountains, dating from as early as 600 or 610 Ma. The Twitya fossils are simple cup-shaped animals, possibly similar to the sea anemones of today. More complex forms tentatively attributed to the Pennatulacea (sea pens) are members of the Ediacaran assemblage which persisted into the Cambrian, noteably at Booley Bay, in Ireland. Some other, more problematic, Ediacarans such as Kullingia also persisted through the Cambrian. (Read more about Ediacaran Survivors.)
Fossil ctenophores are seldom reported. Perhaps the earliest are the two Early Cambrian Chengjiang forms, Xianguangia sinica Chen & Erdtmann 1991 and Maotianoascus octonarius Chen & Zhou 1997. Form the Middle Cambrian comes Xanioascus canadensis Conway Morris and Collins 1996, known from the oldest of the Burgess Shale communities, found on Mt. Stephen in the Kicking Horse Shale Member. A slightly younger community from the Raymond Quarry Shale Member, includes Ctenorhabdotus capulus Conway Morris and Collins 1996. Fasciculus vesanus Simonetta and Della Cave 1978 also occurs, not I have not learned where it sits in terms of age.
Gastropods, having planispiral shells with a wide, expanded aperture, make their first appearance in the Lower Cambrian and seem to have been immediately and markedly successful. The earliest gastropods, such as Coreospira and Heliconella, are normally classified with the suborder Bellerophontacea although (a) they are not typical members of the suborder and (b) some authorities regard the Bellerophontacea as monoplacophorans rather than gastropods.

Although the bivalves did not become common until Arenig (Ordovician) times, their minute, earliest representatives include Pojetaia, from the Lower Cambrian of Australia (Runnegar & Bentley 1982) and Tuarangia, from the Middle Cambrian of New Zealand (McKinnon 1982). The former somewhat resembles the Ordovician palaeotaxodonts, and may have been an ancestor. Another Middle Cambrian form, Fordilla, is probably also a bivalve, though there is some debate on this point.

(After Clarkson 1993, Ch. 8, pp. 191-253.)

The fossil record of annelids is poor: Ediacaran forms such as Dickinsonia and Spriggina have been variously interpreted as annelids, but at least the latter has also been proposed as an arthropod and the best that can be said is that such interpretations are speculative. The earliest undoubted annelid fossils are Lower Cambrian, e.g. from the Chengjiang lagerstätte. The evolution and phylogeny of the annelids is also poorly known; most authors now treat the oligochaetes as a (paraphyletic) stem group.
Arthropods probably existed before the Cambrian; Ediacaran organisms such as Diplichnites and Parvancorina are thought to have been arthropods. In the very basal Cambrian Dengying Formation (central Guizhou, China) the microfossil Markuelia secunda is thought to represent an arthropod embryo (Bengtson & Zhao 1997). By Chengjiang times, arthropods had diversified to include more than twenty five genera, including many of the exotica found in the much younger Burgess Shale, so thoroughly popularised by Gould 1989.

Stem-group arachnomorphs, such as the Nectaspida, had become both widespread and diverse.

Best known of all the extinct arthropods, the trilobites were firmly established by this time, although their origins remain obscure.

Today the Onychophorans are a special though almost insignificant arthropod subphylum represented by the terrestrial Peripatus and it’s allies. In the Cambrian, however, the Onychophorans were exclusively marine and represented by a number of taxa which were common at the time. Microdictyon is notable because its disarticulated plates are a common component of the so-called small shelly fauna. (Read more about Onychophorans.)

(A) Parvancorina minchami (38329 bytes)     (B) Redlichia chinensis (21254 bytes)

Fig. 4: (A) Parvancorina minchami – A candidate Ediacaran arthropod, possibly a trilobite (see Fortey et al. 1996). In this scenario, the central axial ridge and the strongly arched anterior ‘lobes’ may be analogous to the midgut and gastric diverticulae. The scale bar is in centimetres. [Image and interpretation courtesy of Chris Nedin, Department of Industry, Science and Resources, Canberra.]

(B) Redlichia (Hunanolenus) chinensis – Cambrian. Specimen from Henan Province, China. Length approximately 6 cm. [Original image.]

The earliest known brachiopods occur in Lower Cambrian sediments and are already quite diverse. Three classes are recognised; and some authors consider the group to be polyphyletic. The Lingulata are a class of phosphatic-shelled inarticulate brachiopod; the genus Lingula arose in the Cambrian and persists today (possibly the best example of bradytelic evolution there is). The other classes, Articulata and Inarticulata, possess calcitic shells and are probably more closely related to each other than either is to the Lingulata. The earliest articulate group to evolve was the order Orthida. The earliest inarticulates may have appeared in the Middle Cambrian, but the group did not radiate substantially until the Middle Ordovician.
Echinoderms have a very extensive fossil record of about 13,000 species. Possible Vendian members (e.g. Arkarua) have been proposed from the Ediacaran fauna, but the group did not radiate strongly until the Cambrian. Cambrian representatives already possess ambulacral plates, indicating the presence of a water-vascular system and tube feet. Some of these early forms did not exhibit the pentaradial symmetry for which the group is now best known. The echinoderm classes living today, except the holothuroids, are known from the early Ordovician, while all other classes of the Cambrian radiation became extinct during the Paleozoic. (After Nielsen 2001, pp. 419-420.)
The earliest pterobranchs, including the graptolites, first appear in the Middle Cambrian. Their origins are unknown (Rickards 1979). The oldest fossil graptolites are the benthic dendroids (order Dendroidea; planktonic graptolites did not arise until earliest Ordovician, Tremadoc, time). These were colonial organisms having a complicated construction, and rather long stratigraphic ranges at the generic level. The dendroids persisted until the Carboniferous.
Several genera of chordate were already established by the time of the Early Cambrian deposition at Chengjiang (Conway Morris 2000). The better known Middle Cambrian Pikaia may have been something of a laggard.

“Conodonts were the first vertebrates to produce a mineralised skeleton and species of Protopanderodus and Periodon were some of the first animals to inhabit the deep-sea realm. Crown enamel in Protopanderodus and Periodon records seasonally entrained growth and cyclical alternation of episodes of growth and function. Growth episodes spanned 7-10 days in Protopanderodus graeai and only 2-3 days in Periodonaculeatus. It is postulated the former had a pelagic/nektonic mode of life whilst the latter was nektobenthic and specialised to living in deep-water habitats. The growth of P. graeai is further characterised by two distinct phases, the production of a triangular, symmetrical ‘proto-element’ followed by addition of increments of different thickness to the inner and outer edges. The curved and twisted geometry of the adult element was not initiated until the second phase of growth. This has implications for the function of the proto-element and the mode of life of juvenile animals” (Armstrong & Smith 1999).

The earliest record of a land plant is the Cambrian age Aldanophyton, unfortunately known from only a single occurrence in Russia. The original, and perhaps only detailed description is in Russian; information in English about this enigmatic fossil is difficult to come by.

Extinctions

Extinction rates were generally high throughout the Cambrian, typically between 20 and 30%, but two major peaks in the mid Cambrian (Upper Solvan) and at the close of the Cambrian (Dolgellian) qualify as mass extinctions.

“Three significant paleoceanographic events are juxtaposed in Upper Cambrian (Steptoean Stage) sequences of Laurentia [= Dolgellian of Europe]:

  1. a mass extinction of trilobites that marks the base of the Steptoean (Marjumiid-Pteroceaphliid biomere);
  2. a large positive excursion in carbon isotope values that spans much of the Steptoean (SPICE event); and
  3. an imprecisely dated craton-wide drop in sea level (Sauk II – Sauk III event) that was terminated by widespread flooding in the Late Steptoean (mid-late Elvinia Zone time).

The first two events are clearly global in scope, but the scale and timing of the sea level drop is not known in detail outside Laurentia. We have recently undertaken a project aimed at evaluating whether there are demonstrable cause and effect relationships among these events in widely separated Laurentian sections” (Saltzman & Runkel 2001).

(Read more.)

Conclusion

The Cambrian was a time of mild, equitable climate, when high sea levels covered extensive continental margins. Sediments deposited on the margins of one of the Cambrian continental masses, Avalonia, now form the type section in Wales. Modern dating techniques have shown that the Cambrian was of shorter duration than previously thought: the currently accepted duration is 53 Ma, from 543 to 490 Ma ago.

The period occupies a special place in the study of paleontology, because it is in the sediments of Cambrian age that fossils ‘suddenly’ become common for the first time. This effect has come to be known as the “Cambrian Explosion.”

Most modern phyla capable of leaving fossils are known from the Cambrian, though, numerically, fossil collections from this interval are dominated by arthropods. Best known of these are certainly the trilobites. Much of our knowledge of early metazoan evolution has come from the study of three remarkable lagerstätten – Sirius Passet in northern Greenland, Chengjiang in southern China, and the Burgess Shale of western Canada.

Extinction rates were generally high throughout the Cambrian, with two major peaks qualifying as mass extinctions.

References

Armstrong, Howard A.; Smith, Caroline J. 1999: Environmentally Entrained Growth in the Earliest Vertebrate Teeth. Palaeontological Association 43rd Annual Meeting, University of Manchester, 19-22 December 1999 (Oral Presentation)

Arthur, Wallace 1997: The Origins of Animal Bodyplans. Cambridge Press, London.

Bengtson, S.; Conway Morris, S.; Cooper, B.J.; Jell, P.A.; Runnegar, B. 1990: Early Cambrian Fossils from South Australia. Mem. Assoc. Austral. Paleont. 9, 364 pp.

Bengtson, Stefan; Zhao, Yue 1997: Fossilized Metazoan Embryos from the Earliest Cambrian. Science 277: 1645-1648.

Briggs, Derek; Erwin, Douglas; Collier, Frederick 1994: Fossils of the Burgess Shale.

Chen, Jun-Yuan; Oliveri, Paola; Li, Chia-Wei; Zhou, Gui-Qing; Gao, Feng; Hagadorn, James W.; Peterson, Kevin J.; Davidson, Eric H. 2000: Precambrian Animal Diversity: Putative Phosphatized Embryos from the Doushantuo Formation of China. Proceedings of the National Academy of Sciences of the USA 97: 4457-4462.

Clarkson, E.N.K. 1993: Invertebrate Paleontology and Evolution (3rd ed.) Chapman and Hall.

Conway Morris, Simon 1998: The Crucible of Creation. Oxford.

Conway Morris, Simon 2000: The Cambrian "explosion": Slow-fuse or megatonnage? Proceedings of the National Academy of Sciences of the USA 97: 4426-4429.

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